1. Field of the Invention
The present invention relates generally to colorimetry, more specifically to color hard copy apparatus, and, more particularly, to an apparatus for color measurement using a physically distributed multiplicity of sensors.
2. Description of the Related Art
Color hard copy apparatus are commercially available for printing color images from digital alphanumeric text and graphic or photographic image data. While the data source may have excellent color gamut representation capability, the printed images produced can be affected by unpredictable variations in the writing instruments, the print medium, and the ambient atmospheric conditions. For example, in an ink-jet printer, inconsistencies of ejected ink drop size (volume generally measured in picoliters), ink dot interactions on the media, ink-media interactions, ink chemical composition from batch-to-batch, and local temperature and humidity can result in a printed color not closely matching the desired color as represented by the data.
Digital color print quality benefits greatly when color calibration techniques are employed. By measuring the actual color produced as well as knowing the desired color, it is possible to compensate for any recognizable difference between the two. Printing commands using correction factors can be modified in real time such that the commanded color deposition exactly, or at least within visual discrimination capability, matches the desired color.
Colorimetry is “any technique by which an unknown color is evaluated in terms of standard colors; the technique may be visual, photoelectric, or indirect by means of spectrophotometry.” McGraw-Hill, Dictionary of Scientific and Technical Terms, Fourth Edition, Copr. 1989. A variety of apparatus for making color measurements exist. For example, an external instrument can be used to read a test print and the derived data can be analyzed for determining such correction factors. Spectrophotometers are highly accurate, but generally too expensive to integrate into a commercial printer, particularly common desktop computer peripherals and multifunction (printer/copier/facsimile) office products. Spectrophotometers use a broadband (wide range of frequencies) light emitter and sensor, relying on complex wavelength discriminating optical components (e.g., variable wavelength filters) to accurately measure color properties. The broadband spectrum is spit into multiple paths for detection at each of the desired wavelengths, or multiple filters are moved into the optical path from the observation spot to a single detector. Generally, thirty or more channels are being measured and a large effort is required to focus all channels on a small spot of color print. Note that in general, external instruments are inconvenient, requiring separate power, alignment apparatus, a user interface for communicating results to the hard copy apparatus, and user familiarity with calibration operations.
Lower cost type instruments, such as a densitometer or a colorimeter, can be employed. As examples, a densitometer device using a changeable filter arrangement, is disclosed by Lloyd et al. in U.S. Pat. No. 5,508,826, METHOD AND APPARATUS FOR CALIBRATED DIGITAL PRINTING USING A FOUR BY FOUR TRANSFORMATIO MATRIX (assigned to the common assignee herein); Haraguchi et al. in U.S. Pat. No. 6,002,498, describe a conversion function for obtaining analytical density for spectral density using a densitometer in an ink-jet printing apparatus.
Another prior art solution is to measure color patches with a multi-light-emitting-diode (LED) device. Intermediate accuracy levels are achieved by measuring the spectral characteristics of color test patches in more than two or three bands. LED devices have a drawback in not providing optical energy at short wavelengths. The LED's must illuminate the same, relatively small, color test patch, imposing unnecessary design constraints. Moreover, the effect of fluorescing “brighteners” that media vendors incorporate into their paper products is not detectable.
Kent Vincent, in U.S. Pat. No. 5,671,059 (assigned to the common assignee herein) for an ELECTROLUMINESCENT COLOR DEVICE, has multiple electroluminescent emitters and a broadband sensor forming a colorimeter. The emitters produce light covering spectral segments determined by a filter layer and an active layer wherein the spectral segments sufficiently cover the visible spectrum to enable a wide range of colors to be accurately measured.
Another prior art method is described by Hayasaki in U.S. Pat. No. 6,042,213 for a METHOD AN APPARATUS FOR CORRECTING PRINTHEAD, PRINTHEAD CORRECTED BY THIS APPARATUS AND PRINT APPARATUS USING THIS PRINTHEAD. A CCD camera and image processing is used during printhead manufacture to provide correction data for adjusting the amount of ink discharged from each nozzle.
Another prior art method is describe by Chan in U.S. Pat. No. 5,107,332 for a METHOD AND SYSTEM FOR PROVIDING CLOSED LOOP COLOR CONTROL BETWEEN A SCANNED COLOR IMAGE AND THE OUTPUT OF A COLOR PRINTER (assigned to the common assignee herein and incorporated herein by reference). A digital data processing method for continuously correcting for error in color output from a scanned image is described.
There is a need for an accurate, inexpensive, on-board, color measurement apparatus for measuring actual color produced by a hard copy apparatus on currently used media so that compensation for the difference between actual and desired values can be made with digital data processing techniques to modify printing commands accordingly and make real time corrections.